test(screenshot): avoid bare await statements in the guard tests

CodeQL's py/ineffectual-statement flagged `await pending` and `await task`
as statements with no effect: it does not model an await of a bare name as
side-effecting, only an await of a call. Both now bind or assert on the
result, which is also a stronger test -- take_snapshot returning None is
asserted rather than assumed.

This is the CodeQL Code Quality (python) gate failure on the previous head.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012Nm7tyA1nfxNCWFXaR3AxV

Author: kingpanther13

tests/src/unit/test_dashboard_screenshot_theme_guard.py

@@ -538,7 +538,9 @@ async def test_overlapping_batches_are_serialized_per_engine_user(self) -> None:
 
         _FakeWsClient.user_data[THEME_USER_DATA_KEY] = dict(_CLOBBERED_THEME)
         await first.restore()
-        await pending
+        # Bound to a name: a bare `await <name>` statement is what CodeQL's
+        # py/ineffectual-statement flags.
+        assert await pending is None
         await second.restore()
 
         assert _FakeWsClient.user_data[THEME_USER_DATA_KEY] == _DARK_THEME
@@ -602,7 +604,7 @@ async def hang(_ws: Any) -> Any:
             await asyncio.wait_for(started.wait(), timeout=1)
             task.cancel()
             with pytest.raises(asyncio.CancelledError):
-                await task
+                _ = await task
 
         _FakeWsClient.user_data[THEME_USER_DATA_KEY] = dict(_DARK_THEME)
         other = ThemeGuard.for_capture(_PUPPET_CREDENTIAL, None)